This invention relates to power tools and particularly to hammer mechanisms for such power tools.
It is known to provide a hammer mechanism for a power tool comprising an axially movable chuck spindle shaft journalled for rotation in a housing, a rotary ratchet fixed on the shaft, a fixed ratchet in the housing and means selectively to keep the ratchets apart or permit them to engage on user pressure on the shaft. When engaged, as well as the rotary motion imposed by a drive for the shaft, a reciprocating action is imposed on the shaft.
Where the power tool is a drill and a chuck mounted on the chuck spindle shaft is fitted with an appropriate tool bit, the reciprocating (hammer) action greatly improves drilling performance in materials such as masonry. However, the reaction of the reciprocation of the shaft is transmitted to the housing and this is felt by a user as undesirable vibration. On the other hand, in very tough materials, cutting performance can be directly related to the pressure imposed on the housing by the user, and so the user has direct control of the performance of the drill.
It is also known to isolate the fixed ratchet from the housing via a spring, so that the reaction of the ratchet is absorbed by the spring. This not only cushions the impact for the user, so that the tool does not exhibit so much vibration, but also the energy of the reaction is stored in the spring and reimparted to the rotary ratchet on return of the spring. In some soft materials the cutting performance is actually improved by the spring because its reaction time is shorter than the inertia of both the tool and user allows.